The HST calibration of SNe Ia luminosities and
the large scale value of H0
Lukas Labhardt1, A. Saha2, Allan Sandage3, G. A. Tammann11 Astronomisches Institut der Universität Basel,
Venusstrasse 7, CH-4102 Binningen, Switzerland 2 Space
Telescope Science Institute, 3700 San Martin Drive, Baltimore,
Maryland 21218 3 Observatories of the Carnegie Institution of
Washington, 813 Santa Barbara Street, Pasadena, CA 91101
Abstract
Type Ia SNe are potentially valuable standard candles, as indicated by
Kowal's (1963) first Hubble diagram of SNe of type I. A modern version
of this Hubble diagram was presented by Sandage & Tammann (1993,
1996), based on a complete fiducial sample of 56 blue SNe Ia with the
following properties:
type Ia classification is ascertained from spectroscopy or
because the SNe occurred in galaxies of type E or S0;
their maximum magnitudes Bmax and Vmax must be reasonably
well determined;
their color at maximum lies in the range
-0.25 <=(Bmax-Vmax) <=0.20 - after correction for Galactic
absorption. This is to minimize the effect of internal absorption and to
guard against spectroscopically peculiar SNe Ia like SN 1986G and 1992K,
which are intrinsically red and underluminous;
the recession velocity v of the parent galaxies is large enough
to avoid substantial effects of peculiar motions and below an upper limit
to minimize non-linearity effects, i.e. 3<logv<4.5.
The resulting Hubble diagram demonstrates that SNe Ia are powerful
standard candles at maximum light. A linear fit of slope 0.2,
corresponding to linear expansion, to the SNe Ia data yields an
overall rms scatter in apparent magnitude of <=0.35 mag.
In order to calibrate the absolute magnitudes of SNe Ia at peak
brightness, i.e. establish the absolute scale from a primary standard
candle, an HST program was mounted to calibrate the luminosities of
a few nearby SNe Ia by obtaining Cepheid distances to the parent
galaxy. We used the pre-refurbished HST to find Cepheids with
MV>-4 mag (corresponding to periods > 10d) in IC 4182
(SN 1937C) and NGC 5253 (SNe 1972E and 1895B) and - after repair of
HST - in NGC 4536 (SN 1981B), NGC 4496A (SN 1960F), and NGC 4639
(SN 1990N). A seventh SN Ia, SN 1989B, has occurred in NGC 3627, a
bona fide member of the Leo group; another member of this group is
NGC 3368 whose HST Cepheid distance has been determined by Tanvir et
al. (1995). The absolute B and V magnitudes of the seven
calibrators agree within the observational errors and give the mean
values
Combining these mean absolute magnitudes with the Hubble diagram
mentioned above gives a mean Hubble constant of
H0=58+7-8 km s-1 Mpc-1 (external error).
Full details of our analysis - including a discussion of
second-parameter corrections which affect the value of H0 by
less than 10% - are given in
Saha et al. (1997).